Time Scale - after Chris Rowan
Chris at
Good Schist is hosting this month’s
Accretionary Wedge #13:
Geology in Space.
I think that I should preface this piece by saying that I’m not a planetary scientist and this just my understanding, and I apologise for any inaccuracies in advance. If you stumble across this piece via a search engine and want too use this for research / an essay may your god help you.
Something catastrophic may (or may not*) have happened during the Cambrian on Venus.
The timescale (adapted from Chris Rowan at Highly Allochtonous) on the left shows the sort of time I’m dealing with here, about 500 Ma ago.
The surface ages of a planetary body are dated by crater counting, the older the surface, the more the craters (and the larger the craters) are. By this measure, Venus appears to have an usually uniform crater density and most craters are fresh and little degraded implying that Venus underwent a planet-wide resurfacing episode at about 500 Ma. The easiest way to resurface a planet is to flood it with volcanic lava, 80% of the plains of Venus are lava flows and there are over 50,000 volcanoes in nearly 650 volcanic fields mapped on the surface of Venus. The large volcanic structures appear to post-date the resurfacing – yet no volcanoes are seen erupting today and most would be considered extinct.
Volcanos on Venus (source: USGS)
The implication is that during the Cambrian there was a massive volcanic episode with the outpourings of lava almost completely obliterating what went before, and then the volcanism stopped just as abruptly.
As a further clue to what might have happened we need to look more closely at a couple of other surface features on Venus, the highlands and chasmae.
![NASA]](http://hypocentre.files.wordpress.com/2008/09/venustoponasa.jpg "venus magellan topoography nasa")
Surface features of Venus as radar mapped by Magellan (Source: NASA)
The highlands in this Magellan radar image, colour coded according to elevation are yellow, for example Ovda Regio and Thetis Regio in the centre right of the image. Below Thetis Regio is the circular chasm of Artemis Chasma.
Tessera terrain, Eistla Regio, Venus (Source: Nasa)
The highlands of Venus stand about 2 – 4 km higher than the surrounding volcanic plains and from the crater count they are slightly older. They are dissected by often orthogonal ridges and grooves produced by a complex history of compression and extension, giving rise to what is known as tessera terrain (a tessera being a tile in a mosaic).
In extensional faulting, the separation of the faults is characteristic of the depth of brittle / ductile transition. The scale of the tesserae (about 5-20 km across) can be used to imply a relatively shallow aesthenoshere / lithosphere detachment ~10km, perhaps unsurprising for a hot planet with a surface temperature in the highlands of 374°C (lowlands 465°C).
Coranae and chasma (Source: NASA)
The chasmae on the younger lowlands are associated with the margins corona structures, believed to be large diapiric upwellings. The extension related chasmae, a couple of kilometres deep, have profiles which from modelling of the four main coronae gives lithospheric elastic thicknesses of 15, 40, 37 & 35 km which are comparable to Earth (25 km Hawaii – 50 km subduction zones). These large elastic thicknesses require low thermal gradients (<10°C/km – half the expected) and a thicker lithosphere to when the highlands were created.
Venus Lithosphere Change (Source: Unknown)
So, lithosphere thickness may have changed dramatically with time due to a change in convection by planetary cooling. In this theory highlands are emplaced just before the switch, with a thin lithosphere and prolific magmatism. Volcanic rises are emplaced after the change with thick lithosphere and low magmatism. The change-over appears to be rapid as there are no transitional forms. [Apologies for not linking the source of the above image - if anyone knows it leave a comment].
If this process occurred then is it a process that is common in terrestrial style planet thermal evolution? Could rapid lithospheric thickening happen on Earth changing the tectonic style? Or has it already happened?
[*Note: some more recent authors (e.g. here and here) now dispute the timing of this catastrophe or that there was a massive volcanic outpouring at all. Although they don't seem to preclude a catastrophic event at 500 Ma, models also allow a more steady state resurfacing.]
